1. Field of the Invention:
This invention relates to a method for the surface treatment of a material containing carbon, and more particularly, to a method of forming a layer of the carbide of titanium (Ti), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W) or manganese (Mn), which will hereinafter be referred to as the carbide forming element, on the surface of the material in a fluidized bed furnace.
2. Description of the Prior Art:
A fluidized bed furnace having (as a heat medium) a fluidized bed which is formed by blowing air, argon or other gas into alumina powder is used for the heat treatment of steel. This heat medium enables the uniform and rapid heating of the material to be treated, since it has a uniform temperature distribution and transfers heat rapidly.
An attempt has been made to use such a fluidized bed furnace for the diffusion coating of the metal surface, as will now be described with reference to FIG. 2. A coating agent in the form of a mixed powder composed of a substance containing a cementing metal and an inert substance, such as alumina, is placed on a diffusion plate b in the main body a of a fluidized bed furnace. An inert gas, such as argon (Ar), is introduced into the main body a through a gas supply passage c.sub.1 to fluidize the powder to form a fluidized bed d. The top cover e of the main body a is opened, the material f to be treated is buried in the fluidized bed d and the cover a is closed. The utmost care is required to ensure that the furnace is closed tightly. Halogen, vapor, as an activator, is introduced with hydrogen, as a carrier gas into the fluidized bed d through a gas supply passage c.sub.2. The halogen vapor and the powder of the cementing metal react with each other to generate a gas of the halide of the cementing metal. The halide gas is decomposed upon contacting the material f in the fluidized bed d to deposit the cementing metal on the surface of the material.
This method and the furnace used for carrying it out have, however, a number of inconveniences as will be pointed out. Firstly, halogen vapor as an activator is necessary and, hydrogen, use of which is essential for carrying the halogen vapor, is highly explosive and requires a lot of care to ensure the gastightness of the piping and the furnace. The operation of the furnace is, therefore, not always efficient. Secondly, it is impossible to take out the coated material from the furnace while it is still at a high temperature, since the furnace need be kept tightly closed so that no explosion of hydrogen may occur. Therefore, it is difficult to harden the material immediately after it has been coated. Thirdly, an apparatus is required for generating the halogen vapor, thus making the furnace complicated both in construction and in operation.